This invention relates to a ceramic wiring substrate and a process for producing the same. More particularly, the present invention is concerned with a mullite ceramic wiring substrate having a wiring conductor which is dense and exhibits a high metallizing strength and a process for producing the same.
In recent years, with high integration of semiconductor devices, ever-increasing requirements for higher density wiring, higher performance and higher reliability have been made to wiring substrates mounting such devices. Particularly, a great task with respect to wiring substrates used for computers etc. is to attain a higher rate of signal propagation and a higher reliability.
As a ceramic for such wiring substrates, a ceramic composed mainly of alumina (Al.sub.2 O.sub.3) has already been put to practical use. However, such a ceramic is accompanied by a fundamental problem that the rate of signal propagation is low due to a high dielectric constant of alumina. On the other hand, substrates prepared by using ceramics having a low dielectric constant have recently been developed, and mullite ceramics among them are now attracting a particular attention.
However, the use of mullite ceramics as a wiring substrate brings about the following problems:
(1) Since mullite ceramics hardly react with high-melting metals such as tungsten and molybdenum in a reducing atmosphere, the adhesion between mullite ceramics and these metals are extremely low. In this connection, it is noted that W. M. Kriven and Joseph A. Pask submitted an article entitled "Solid Solution Range and Microstructures of Melt-Grown Mullite" to Journal of the American Ceramic Society, Vol. 66, p. 649. The authors conducted experiments on melting of a pure mullite using a crucible made of molybdenum and proved that molybdenum and the mullite do not chemically react with each other even at a temperature corresponding to or exceeding the melting point of mullite. Since the chemical properties of tungsten are substantially the same as those of molybdenum, tungsten does not chemically react with mullite either. The present inventors also have confirmed experimentally such reactivity between molybdenum or tungsten and mullite, i.e., confirmed that neither molybdenum nor tungsten reacted with mullite at a temperature of 1750.degree. C. or lower.
It is essential for a ceramic wiring substrate that a ceramic is strongly bonded to a conductor on the surface of the wiring substrate, e.g., at signal input and output terminals or IC chip-connecting terminals. In an alumina wiring substrate, such a strong bonding has conventionally been attained by the following method.
Specifically, an alumina wiring substrate having a metallic tungsten conductor distributed thereon is heated to the sintering temperature thereof, causing the melting of a sintering additive which has been added to the alumina. The molten sintering additive penetrates into the voids in the conductive layer made of tungsten in the next stage. By penetration of such a molten sintering additive, tungsten can, for the first time, be integrated with alumina, thus leading to the formation of a strong bonding therebetween.
As mentioned above, a mullite ceramic chemically reacts with neither tungsten nor molybdenum. But, it is expected that a mullite ceramic could be strongly bonded to a conductive metal through proper selection of a sintering additive capable of penetrating into a conductor layer made of molybdenum or tungsten, as in the above-mentioned alumina.
However, in conventional mullite substrates, a sintering additive which has been added to the mullite ceramic does not exhibit sufficient melting and penetration into a conductive metal layer at its sintering temperature. In order to attain the penetration of sufficient amount of the molten sintering additive into the conductive metal layer, it is necessary for the surface of the conductive metal to be wetted with the molten sintering additive. The angle between a solid 1 and a liquid 2 (contact angle .theta.) as shown in FIG. 1 is ordinary used as a measure of wettability. In general, when the angle .theta. is greater than 90.degree. C., the solid is considered not to be wetted while when the angle .theta. is smaller than 90.degree. the solid is considered to be wetted. For mullite ceramics, an angle .theta. smaller than about 10.degree. is required for bonding a ceramic to a conductive metal in a manner similar to the permeation of a sintering additive in the case of an alumina substrate.
However, the contact angle .theta. between tungsten or molybdenum and a sintering additive which has been employed in the conventional mullite ceramics is relatively large, i.e., about 20.degree. to 40.degree.. Therefore, it is difficult for a sufficient amount of the sintering additive to penetrate in a molten state into a conductive metal layer distributed on the mullite ceramic substrate, which causes lowering in the strength of bonding between the mullite and the conductive metal or causes scattering of bonding strength.
(2) When heated to a sintering temperature of the mullite ceramic substrates, the mullite ceramic is densified sufficiently, whereas metals having a high melting point, such as tungsten or molybdenum, is not densified sufficiently and still has a relatively porous microstructure.
If the conductive layer formed on the surface of the wiring substrate is porous, serious problems arise during subsequent treatments of the substrate. For example, during a plating treatment of the substrate, a plating solution permeates into the conductive layer to cause swelling, peeling, oxidation and discoloration of the film formed by plating. Therefore, the conductor layer formed on the surface of the ceramic substrate must be dense to such an extent that the plating solution cannot permeate thereinto.
However, since in conventional mullite substrates, the sintering additive does not sufficiently penerate into the conductor layer at the sintering temperature, as mentioned above in item (1), pinholes are present particularly in the conductive layer formed on the surface of the mullite ceramic, which renders the mullite substrates water absorptive.